Support for screenshot like XSS hunter

[hackncode]

https://xsshunter.com/features

Using HTML5 canvas API for self hosted instances.
If possible, for shared public instance.

[geeknik]

Please allow us to toggle this on and off because some programs don't allow the taking of screenshots.

[Mzack9999]

Implemented via #53

The client side code injected via the scripting engine should use https://github.com/niklasvh/html2canvas and post the result back to the interactsh self-hosted instance. XSSHunter implements a fully working payload at https://your_handle.xss.ht.